Sweet corn is very susceptible to seed rot and damping-off caused by soilborne plant pathogenic fungal species. Corn is particularly susceptible to disease caused by members of the genus Pythium. Seed rot and damping-off cause decreased emergence of plants, reduced plant vigor, and plant death. Consequently, these diseases result in crop loss and dramatically reduced yields. Therefore, most commercial seeds of corn hybrids are treated with antimicrobial agents to protect against seed rot and damping-off.
The application of the chemical fungicide captan (N-(trichloromethylthio) tetrahydrophthalimide) to corn seed has been found to be effective in enhancing seed germination and protecting seedlings against Pythium and Fusarium species, causal agents of seed rot and damping-off. Captan has been banned for use in agriculture in Europe because it is a known carcinogen. Currently, agricultural uses of captan are permitted in the United States. Other seed-applied chemical fungicides in common use include Apron and Maxim. These fungicides are expensive and have a limited range of effectiveness. The effectiveness of Apron is essentially limited to control of Pythium, whereas Maxim is primarily effective in controlling Rhizoctonia.
A growing awareness that agricultural practices have a great impact on human health and on the environment has spawned research into the development of effective biological control agents, or biocontrol agents, to protect crop plants against disease. In general, a biocontrol agent is a living organism that controls disease. Biocontrol agents offer important advantages over chemical pesticides. The use of these agents in agriculture introduces fewer risks to human health and to the environment than chemical pesticides. Additionally, biocontrol formulations are generally relatively inexpensive to produce.
Certain bacterial strains isolated from plants have been found to protect plants against the devastating effects of diseases caused by plant pathogenic fungi. U.S. Pat. No. 4,588,584 to Lumsden, et al. discloses a strain of Pseudomonas cepacia that is effective in controlling Pythium diseases of cucumber and peas. U.S. Pat. No. 5,244,658 discloses another bacterial strain, designated Pseudomonas cepacia AMMD (ATCC 53795), that is effective in the control of Aphanomyces, a fungus that causes root rot in peas. Recently, the species Pseudomonas cepacia has been reclassified and is properly referred to as Burkholderia cepacia. Accordingly, this species will be referred to herein as "Burkholderia cepacia", or "B. cepacia".
Research has been conducted to evaluate the efficacy of B. cepacia AMMD (ATCC 53795) in protecting sweet corn against Pythium seed rot and damping-off disease. Bacteria were applied to seeds as a slurry at a concentration of approximately 10 million bacteria per seed. Treatment of seeds with B. cepacia AMMD was found to enhance emergence, which is expressed as the percentage of seeds that develop into seedlings. Emergence was significantly higher for seeds treated with B. cepacia AMMD (approximately 67%) as compared to untreated seeds (approximately 43%). However, a high percentage of corn plants grown from seeds treated with B. cepacia AMMD exhibited chlorotic streaking. The frequency of chlorotic streaking increased as the amount of bacteria applied to the seed was increased. The incidence of chlorosis was higher in plants grown from seeds treated with both captan and B. cepacia AMMD.
What is needed in the art are bacterial strains that are effective in protecting sweet corn against seed rot and damping-off without causing undesirable side effects, such as chlorotic streaking.